Mr. McAuliffe – a former chairman of the Democratic National Committee and longtime friend of Hillary and Bill Clinton — won the endorsement of Boyd Marcus, who has handled GOP campaigns for governor of the commonwealth and U.S. senator.

Although Mr. Marcus is not well known to the general public, he knows almost every town in Virginia and its voter base, and has many connections to state power brokers and Richmond moneymakers. He is joining the McAuliffe team as a paid adviser, the campaign confirmed.

The Virginia governor’s election is viewed by many Republicans as the top off-year race, and they want to keep the statehouse, now occupied by embattled Gov. Bob McDonnell.

In a statement released by the McAuliffe campaign, Mr. Marcus said that “Virginia needs an experienced businessman who will put the practical needs of our people ahead of political ideology.” And he added: “I’ve never before supported any Democrat, but this election Terry is the clear choice for mainstream conservatives.”

Cuccinelli campaign spokesman Richard T. Cullen said that “Virginians are concerned about each candidate’s vision for the future and how they plan to grow the economy and create jobs, not who their political consultants are.” Mr. McAuliffe, he said, “has misled Virginians at every turn about his business record.”

Mr. McAuliffe is a top GOP target partly because of his long association with former President Clinton and Mrs. Clinton, the former secretary of state.

Polls show Mr. Cuccinelli is popular with evangelicals and social conservatives, but some Republicans considered him too extreme to win. Mr. McAuliffe has been running relatively even with Mr. Cuccinelli, Virginia’s attorney general.

Larry Sabato of the University of Virginia, a top political scientist, said the Marcus move is indicative of the split in the GOP. “The moderates just won’t go for Cuccinelli,” he said.

The McAuliffe campaign applauded Mr. Marcus, saying in a statement, “Boyd is a great addition to our campaign team that is focused on bringing together Virginians of both parties.”

In addition to his campaign work, Mr. Marcus was chief of staff to former Gov. James Gilmore, and later chief of staff to U.S. Rep. Eric Cantor of Virginia, now majority leader for the House of Representatives.

Earlier this year, Mr. Marcus backed Lt. Gov. Bill Bolling, a more centrist candidate, for the Republican nomination. But after the Republican Party in Virginia switched from a primary to a nominating convention, which would attract more of the party’s conservative base, Mr. Bolling dropped his bid. He has declined to endorse Mr. Cuccinelli.

In 2009, Mr. Marcus ran the failed campaign of another Republican who opposed Mr. Cuccinelli for the party’s nomination for attorney general.

About Washington Wire

Washington Wire is one of the oldest standing features in American journalism. Since the Wire launched on Sept. 20, 1940, the Journal has offered readers an informal look at the capital. Now online, the Wire provides a succession of glimpses at what’s happening behind hot stories and warnings of what to watch for in the days ahead. The Wire is led by Reid J. Epstein, with contributions from the rest of the bureau. Washington Wire now also includes Think Tank, our home for outside analysis from policy and political thinkers.